Wanted to post the exact same thing. Experienced that a few months ago. Absolutely horrible. And i think the doctor's instructions made it worse. They told me to slump together and not round out my back. But after several failed attempts I rounded out my back and it finally went in. But that's only the beginning. The sucking sensation was so awful. It felt like they were trying to drain my brain.
Yeah they had me round my back, were pretty clear about it, but this was 30+ years ago so maybe some medical knowledge has come around, but I've not heard of it.
Sorry you had to go through it, sure wish they had a better way to get spinal fluid for tests.